The CMS Tracker is the world's largest silicon detector. It has only recently been moved underground and installed in the 4T solenoid. Prior to this there has been an intensive testing on the surface, which confirms that the detector system fully meets the design specifications. Irradiation studies with the sensor material shows that the system will survive for at least 10 years in the harsh radiation environment prevailing within the Tracker volume. The planning phase for SLHC as the successor of LHC, with a ten times higher luminosity at the same energy has already begun. First R\&D studies for more robust detector materials and a new Tracker layout have started

Simonis, H J

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Available on CMS information server CMS CR-2008/008The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report9 April 2008Status of the Silicon Strip Detector at CMSH.J. Simonis, On behalf of the CMS Tracker CommunityAbstractThe CMS Tracker is the world’s largest silicon detector. It has only recently been moved undergroundand installed in the 4T solenoid. Prior to this there has been an intensive testing on the surface,which confirms that the detector system fully meets the design specifications. Irradiation studies withthe sensor material shows that the system will survive for at least 10 years in the harsh radiationenvironment prevailing within the Tracker volume. The planning phase for SLHC as the successor ofLHC, with a ten times higher luminosity at the same energy has already begun. First R&D studies formore robust detector materials and a new Tracker layout have started.Presented at 10-th INTERNATIONAL CONFERENCE ON INSTRUMENTATION FOR COLLIDING BEAMPHYSICS,February 28 - March 5, 2008, Novosibirsk,Russia1 IntroductionThe CMS Tracker project was inaugurated more than ten years ago. Only recently the completed Tracker has beeninserted underground into the center of the CMS magnet, hence marking the turning point from the constructionphase to the actual experiment.According to the CMS philosophy, the Silicon Strip Tracker is designed as a compact, hermetically closed system.Each track is recorded with at least 10 high resolution space points in the range ‖η‖ < 2.4 [1] [2].The LHC environment, namely the short (25ns) bunch structure and the high luminosity of 1034(s · cm2)−1 ledto the chosen high granularity of 10M readout channels and special radiation hard 〈100〉-Si sensor material. TheTracker will be operated at a temperature of −10◦C to reduce the irradiation induced leakage currents and topermit a stable 10 years of operation.2 ConstituentsIP?z (mm)BeamTOB              TECTIB   TIDIP 200             1000        2000             2800  z( )r (mm)100050000.1                                    1.0                      1.5   1.82.5?Figure 1: Schematic view of a sector of the Tracker. Each line represents a detector module. The structure is meant to berotated around the beam axis and mirrored at the interaction point.The Silicon Strip Tracker consists of 15148 silicon strip detector modules which are grouped in four large sub-structures: the Tracker Outer Barrel (TOB), the Tracker Inner Barrel (TIB), the Tracker Inner Disk (TID) and theTracker EndCap (TEC) (see fig. 1).2.1 The BarrelsFigure 2: Tracker Inner BarrelIn the two barrel substructures the strips of the modules are oriented parallel to the beam and the prevailing 4Tmagnetic field. The modules are arranged in four (TIB) respective six (TOB) concentric cylinders “shells” aroundthe beam axis, covering the radial range 25cm < r < 108cm.1Two of these layers in TID as well as in TOB are “stereo layers”: here modules are mounted in back-to-back pairs,where one module has an axial strip orientation and the second module oriented at an angle of 100mrad withrespect to the first. The resulting strip crossing leads to an enhancement of the spacial resolution along the stripdirection by about two orders of magnitude.The modules for the TIB subsystem are directly mounted on carbon fiber half shells – two of which combine to afull shell. The four shells of the inner barrel are assembled together and constitute an independent detector unit inthe sense of cooling, powering and readout. Figure 2 shows how the TIB modules are mounted so that they overlapon their shells.For the Outer Barrel system the modules are installed on “rods” (fig. 3), which in turn are used to populate the sixlayers. A rod is a carbon fiber frame equipped with a cooling tube and a pcb motherboard to enable the operation ofsix modules (12 modules for stereo layers). The TOB consists of 688 rods mounted in a large carbon fiber supportstructure.TOB - Rod TEC - PetalFigure 3: Carbon fiber support structures for TOB and TEC: (left) a rod in three different phases of construction; (right) a fullyassembled petal.2.2 The DisksIn the TID and TEC substructures (fig. 1) the modules are oriented perpendicular to the magnetic field. They arearranged in seven rings (three rings for TID) at different overlapping radii around the beam axis. All strips pointradially to the beam axis except for the stereo modules in three rings (numbers #1, #2 and #5). In these casesthe strips of one module are rotated by an angle of 100mrad with respect to its back-to-back partner. The radialpointing is achieved through a wedge shaped sensor design. Keeping the width/pitch ratio for all strips constant,the signal-to-noise value for particle hits remains independent from the position along the strip. Each ring requiresits own sensor, design which led to a large number of different sensors and modules (fig. 4).On the three TID disks, the modules are mounted in full rings on carbon fiber support structures, three rings perdisk.A Tracker EndCap (fig. 5) consists of 9 large disks. Modules are not directly mounted to these disks, but tointermediate structures called “petals” – wedge-shaped portions of the disk. The production of these structuresallowed a reasonable distribution of assembly tasks among the TEC community (fig. 3). Sixteen petals are requiredfor one disk. Eight different types of petals are needed with different numbers of modules, depending on thedistance from the interaction point; e.g. the farthest disk need not be equipped with the three inner rings to coverthe region |η| < 2.4 (fig. 1).After completion and individual testing of these four Tracker substructures, the Tracker was assembled within analuminum support tube which provides thermal insulation in addition to mechanical stability. Equipped with aninsulating foam with cooling pipes on the inner side and an electrical heating outside it acts as a thermal shieldagainst the neighboring electromagnetic calorimeter, which requires a temperature of 18◦C, whereas the siliconsensors of the Tracker are cooled down to −10◦C.2Figure 4: Many different module geometries are needed in the different substructures of the Tracker.Figure 5: One Tracker EndCap fully equipped with petals.3 PerformanceDuring all steps of construction every individual part has been carefully tested. Every detector module as wellas the larger structures – rods, petals, half shells, TID disks – have been operated and thermally cycled to ensurereliability when used in the collision hall.Moreover, a large effort has been made with prototypes from both barrel substructures and the endcap to study thesystem performance in the realistic environment within the 4T magnet during the Magnet Test period, which tookplace in the summer of 2006 in the CMS surface hall. [3].The most conclusive test phase however was the assembly period in the Tracker Integration Facility (TIF), a largecleanroom at CERN, where all the substructures were brought together as a unit.Equipped with about 15% of the final power supplies and readout electronics, a simultaneous study of 25m2 ofsilicon detector area has been done. Cosmic ray data was taken at a rate between 1.5 and 6.5 Hz – depending on theconfiguration of large plastic scintillator plates above and below the Tracker support tube – at different temperaturesdown to (−15◦C). In fig. 6 it can be seen that the average signal-to-noise is about 30 for all four subsystems. Thisvalue is high enough to ensure a good hit efficiency, even when the expected irradiation damage deteriorates theperformance. Further studies about tracking efficiency, alignment, data handling and the reconstruction algorithmshave been done (see refs [4] [5]). The TIF studies demonstrated, that the Tracker has fewer than 0.2% deadchannels and that the expected design performance has been achieved.4 UpgradeDespite the fact that the LHC has not yet delivered a single proton bunch, thinking about an upgrade of theaccelerator and its consequences for the CMS Tracker has already started.30 20 40 60 80 1000100200300 TID Ring 1S/N = 29.4EntriesS/N04008001200 TEC Ring 6S/N = 34.60 20 40 60 80 10001000200030004000TIB Layer 1S/N = 28.50 20 40 60 80 100c02000400060008000TOB layer 5S/N = 31.60 20 40 60 80 100S/NFigure 6: Signal-to-Noise measured for all four subsystems.Normal LHC operation for ten years was one boundary condition in the design of the Tracker. Consequently,after that time the Tracker very likely will have to be replaced due to severe radiation damage accumulated duringoperation.The planned increase of the luminosity by a factor of ten necessitates a complete redesign of the Tracker:1. The granularity has to be increased by a factor of at least five in order to cope with the higher occupancy dueto the much higher number of underlying events per bunch crossing.2. The sensor material has to be more radiation tolerant, especially in the region of small radii (r < 40cm)3. A contribution of the Tracker to the L1 trigger is desirable to reduce the expected much higher data rateto a reasonable level. The only reasonable trigger criterion that the Tracker can provide is the transversemomentum of charged particles. However, the determination of pt in real time requires new electronics thatcan combine the data of two or more neighboring detector layers.4. Another important issue for the Tracker is the reduction of the material budget. The existing Tracker exhibitsalready a radiation length of above 1.6X0 for certain regions of η. The sensor material itself represents onlyabout ≈ 10% of the material budget, so reducing sensor thickness is not a high priority, at least as far as thematerial budget is concerned. On the other hand, there would be a large gain if the material required by theservices could be reduced.The first item can be met with a threefold strategy: for small radii add an additional pixel layer; for large radii useshort strip; and for intermediate radii use “strixels”, a series of very short strips on a sensor. The technical problemof the connection of the frontend electronics could be solved with an additional metallization layer on top of thereadout strips.For the second item much profit can be taken from the results of the RD50 collaboration. There is a clear indicationthat n-on-p magnetic Czochralski silicon is more radiation tolerant than the float zone material used to date. Thiscould be the material of choice for the outer radii (r > 40cm). Only for the innermost region more exoticcandidates like 3D-structures or CVD diamond are under discussion.The third item leads to the wide field of R&D for electronics which is ongoing for all aspects of the module readout:New readout chips with more than 128 channels but less power consumption are desired. Frontend digitizers areneeded as well as logic chips to make trigger decisions. Fast switches are required to get the tremendous amountof data out. Since all these elements are within the Tracker volume, they have to be radiation hard.The fourth item triggers two independent directions of studies:• DC-DC couplers for each module are investigated. Feeding the Tracker with higher voltages could result ina reduction of cables and hence reduce material.• Replace the existing low pressureC6F14 cooling system with a high pressure, two phaseCO2 system, whichis much more efficient. The latter scheme would lead to a significant savings in pipe material. Moreover,the cooling pipes should simultaneously serve as power lines so that the number of reqired cables as well aspipes could be substantially reduced.4R&D studies on all these aspects have only just begun. The necessity for an upgrade of LHC is unquestioned, butstill it is an open question if or how the bunch structure of the accelerator will change. The answer to this questionhas consequences for the new Tracker layout.5 StatusThe completion of the Tracker was a common effort of about 600 people in 50 institutes distributed all over theworld. It took more than ten years from the R&D phase to the end of the construction. Unique methods of qualityassurance had to be established in order to guarantee a uniform behavior of the system. The full Strip Trackerwas completed on surface some month before access was given to its underground destination. This allowed anintensive period of data taking with first performance studies of the system as well as all aspects of data handlingand analysis. It took three months following the insertion of the Tracker (16-dec-2007) to connect all of the opticalfibers, cooling pipes, and cables.Now, while waiting for the accelerator team to finish with machine comissioning, the Tracker is ready to acceptcosmic data, with and without the solenoid field (fig. 7).Figure 7: The Tracker positioned in the center of CMS.References[1] CMS Collaboration, “The Tracker Project Technical Design Report”; CERN/LHCC 98-006 (1998).[2] CMS Collaboration, “Addendum to the CMS Tracker TDR”; CERN/LHCC 2000-016 (Feb. 2000)[3] D. Abbaneo et al; “Tracker Operation and Performance at the Magnet Test and Cosmic Challenge”; CMSNote 2007-029[4] C. Delaere et al; “CMS Tracker commissioning anf first operation experience”; CMS CR 2008-002[5] C. Noeding; “Track Reconstruction and Experience with Cosmic Ray Data in CMS”; CMS CR 2008-0065

Status of the Silicon Strip Detector at CMS

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